EP3268735A1

EP3268735A1 - Non-destructive measurement of the degree of humidity of a material

Info

Publication number: EP3268735A1
Application number: EP16712966.7A
Authority: EP
Inventors: Philippe Mabire; André MAYER; Stéphane HOLE; Thierry Ditchi; Emmanuel GERON; Zoé MOKTHARI
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Pierre et Marie Curie Paris 6; Ecole Superieure de Physique et Chimie Industrielles de Ville Paris
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Pierre et Marie Curie Paris 6; Ecole Superieure de Physique et Chimie Industrielles de Ville Paris
Priority date: 2015-03-11
Filing date: 2016-03-11
Publication date: 2018-01-17
Also published as: FR3033644A1; FR3033644B1; WO2016142634A1

Abstract

The present invention relates to a method of non-destructive measurement of a degree of humidity of a material (B), such as for example a mortar or a concrete in the course of drying, the method comprising: a step of emitting (S1) a radiofrequency signal (S_RF) in the material (B) through an antenna (10a, 10a') embedded in the material, a step of receiving (S2) a reflected signal (SR) in response to the radiofrequency signal emitted (S_RF), a step (S3) of frequency analysis of the reflected signal so as to determine resonant frequencies of the antenna (10a, 10a') in the material (B), and a step of estimating the degree of humidity of the material (B) on the basis of the resonant frequencies determined.

Description

NON-DESTRUCTIVE MEASURE

MOISTURE RATE OF A MATERIAL

Technical field and prior art

The present invention relates to the field of measurements, and relates more particularly to so-called non-destructive measurements for measuring and monitoring the evolution of the moisture content of a material.

The object of the present invention thus relates to a non-destructive measurement technique for measuring accurately and at a lower cost, the moisture content of a material.

The present invention finds many advantageous applications, particularly in the construction industry, and particularly the building industry, by making it possible to determine the moisture content of a solid material comprising one or more inorganic binders such as, for example mortar or concrete.

Examples of applications of the present invention include the construction of structures (bridge, dam, etc.) or buildings (or premises) for residential or professional use, and the development or rehabilitation of said buildings, including the laying of concrete slabs or mortar screeds or smoothing.

Other advantageous applications can also be envisaged in the context of the present invention, for example the measurement and monitoring of the moisture content:

a natural soil (composed for example of silt, sand, clay, etc.), a solid material free of mineral binder and comprising one or more organic binders (for example a gelatinous body, a derivative of starch, etc.), of a composite solid material obtained by sintering (for example a ceramic object),

pulverulent material (for example flour, sawdust, etc.), wood in the form of, for example, chips, granules or platelets, or

of a liquid material.

By moisture content of the material in the sense of the present invention, it is to be understood here throughout the present description which follows the water content of the material, that is to say the amount of water contained in the material. Concrete is a composite building material made from aggregates (gravel greater than 1 cm) agglomerated with a powdery mineral binder (eg cement, lime or calcium sulphate).

The mortar is also a composite building material made from granules (chippings smaller than 1 cm) agglomerated with a powdery mineral binder similar to that of a concrete.

Generally, whether it is a concrete or a mortar, the mineral binder is hydrated, that is to say put in contact with water during a mixing process called "mixing", to allow the agglomeration of aggregates.

We speak of concrete (or mortar) based on hydraulic binders.

Generally, during the physical and chemical processes that follow the contacting of the material (concrete or mortar) with the mixing water, part of this water remains bound to the crystals formed from the inorganic binder. We talk about water of crystallization.

Another part of the water, called residual, remains in the free state in the porosity of the concrete (or mortar); this residual water evaporates at least partially with time, giving way to a solid material.

In the building sector, some concrete structures (especially concrete slabs) or mortars (especially screeds or leveling layers) are often required to receive a covering product such as a layer of synthetic resin, a layer of paint, or a layer of laid or glued flooring (wood flooring, tile or plastic flooring).

Before proceeding with the recovery of the concrete (for example in the form of slab) or the mortar (for example in the form of a screed or a leveling layer), it is imperative to ensure that the rate of Residual moisture of the solid material (concrete or mortar) is low enough to ensure good performance.

A drying phase is therefore necessary; it can take several days or even weeks.

It is noted here that the residual moisture content in the concrete (or mortar) corresponds to the ratio between the weight of the free water and the weight of the concrete (or mortar) dry.

This residual moisture content is usually expressed as a percentage; we also talk about dryness rates.

The free water contained in the concrete (or mortar) gradually evaporates during drying; the residual moisture content will therefore change during the drying phase.

Thus, when this humidity level is satisfactory, it is possible to continue the work and apply for example on the concrete slab or the mortar screed (or the leveling layer) the upper layers.

It is understood here that in the construction industry, particularly the building industry, knowing the moisture content of a concrete or mortar being dried is essential to determine the most appropriate time to continue the work. ; knowing precisely this moisture content optimizes the construction time while ensuring the strength and properties related to the mechanical strength of the structure.

The techniques developed so far to accurately determine the moisture content inside concrete or mortar are often so-called destructive measurement techniques.

These destructive techniques require a partial destruction of the material on which the measurement is to be made; they require sample collection.

The operator in charge of the site to control the moisture content of the solid material (concrete or mortar) takes a sample of the material to heart.

Then, it reduces to powder this sample, then makes it interact with for example calcium carbide in a hermetic enclosure (calcium carbide bomb).

This causes a chemical reaction between the carbide and the water contained in the test material sample.

This reaction causes a release of gas which increases the pressure in the hermetic enclosure.

There is therefore a direct link between the quantity of water contained in the sample of concrete (or mortar) taken and the pressure variation under the enclosure.

To determine the moisture content of the concrete (or mortar), the operator therefore measures the pressure variation in the chamber with a barometer; preferably, this barometer is integrated directly into the enclosure.

There are other destructive measurement techniques.

Following a sample, it is possible alternatively to dry in an oven the sample taken (for example at 110 ° C), and then to compare the weight of the sample before and after drying.

Depending on this comparison, it is possible to determine the amount of water contained in the concrete (or in the mortar) before drying.

These different techniques are destructive.

Although tolerable, the sampling necessary to carry out the measurement is unsatisfactory (especially in the construction industry and especially that of the building).

Moreover, it is not really possible with these techniques to test the homogeneity of drying; indeed, this would require the completion of many samples in the book, for example a slab being dried, which would inevitably lead to more destruction.

These destructive techniques also require the relatively long intervention of a qualified technician.

Under these conditions, it is difficult to use these techniques to finely monitor the evolution of the moisture content of a material such as concrete or mortar: these techniques are indeed long, complex and tedious to implement; they are not practiced on site.

There are non-destructive measurement techniques for estimating the moisture content of a material.

However, the non-destructive measurement techniques developed so far are not entirely satisfactory.

The moisture content of a concrete or a mortar during drying can indeed be measured on the surface by a device type moisture meter tip.

Such an apparatus uses the properties relating to the electrical conductivity of the test material.

It is known that the electrical conductivity of concrete or mortar varies depending on the water content: the presence of water in the material promotes the conductivity of electricity in the material.

According to this technique, a voltage is applied between two or more points, or electrodes, brought into contact with the concrete or the mortar.

The electrical resistance measured between the tips then gives information on the amount of water in the concrete or mortar.

With this technique, however, errors are observed in the measurements made. These errors come mainly from chemical reactions between the electrodes and the concrete or the mortar.

There is also a change in percolation between the grains of concrete or mortar.

Moreover, this technique is not satisfactory; in fact, the measured moisture content is that at the surface.

However, the surface of concrete or mortar dries faster than the heart. The surface of the material also has pollutants such as dust and irregularities that can distort the measurement.

The humidity rate that is relevant to take into consideration to continue the work is therefore that inside the concrete (or mortar); that is to say, the heart.

To realize these measures to heart, other techniques exist; however, they are more complex and require the integration of sensors in the material.

In these techniques, it is often the dielectric permittivity of the material to be tested which is the physical parameter used to carry out the measurement.

Indeed, this dielectric constant is of the order of a few units for most materials; it is around 80 for water.

Therefore, the presence of water in a material such as concrete or mortar singularly increases the measured dielectric constant.

The laws of electromagnetism are then used to measure, either by low-frequency capacitive effect or with a resonant cavity at high frequency.

The major disadvantage of the techniques used is the size of the sensors used or their small radius of action when these sensors are miniaturized.

Indeed, as the measurement must be made at the heart of the tested material, a large space necessarily modifies the way the material dries: the measurement is therefore unreliable.

When the sensor is miniaturized, the drying is not disturbed but measurement uncertainties appear from the granular aspect of the concrete or the mortar: the radius of action of a miniaturized sensor is necessarily small.

The response of the sensor therefore differs according to its environment; the answer is different depending on whether it is close to one constituent or another concrete or mortar.

Non-destructive measurement techniques are therefore not satisfactory, particularly for cost reasons (sensors) and for reasons of precision.

DE 20 2013 102 514 U1 discloses a probe emitting a microwave signal within a material and measuring the time of flight of the signal to estimate the dielectric constant of the material. The probe does not constitute an antenna and the estimate of the dielectric constant does not include any frequency analysis.

Given the disadvantages mentioned above for the various techniques deployed so far, the construction industry, particularly the construction industry, prefers to increase the overall duration of the construction of a structure by leaving a margin of safety in the drying time. This, however, generates indirect costs (general duration of the construction site, postponement of delivery, etc.).

Object and summary of the present invention

The present invention aims to improve the situation described above.

One of the objectives of the present invention is to overcome the various disadvantages mentioned above by providing a non-destructive, simple and inexpensive solution for accurately measuring and monitoring the moisture content of a material, particular of a solid material such as concrete, mortar or other materials.

To this end, the object of the present invention relates, according to a first aspect, to a non-destructive measuring method for measuring a moisture content of a material, comprising:

transmitting a radiofrequency signal in the material through an antenna embedded in the material;

receiving a reflected signal in response to the radiofrequency signal transmitted; frequency analysis of the reflected signal to determine resonance frequencies of the antenna in the material;

estimating the moisture content of the material from the determined resonant frequencies.

Thus, thanks to this succession of technical steps, characteristic of the present invention, it is sufficient for the person in charge to measure the moisture content of a material to cooperate with the antenna a meter of the reflectometer type, preferably portable.

Part of the radiofrequency signal transmitted is then reflected back to the measuring apparatus, while another part of this signal is transmitted in the environment of the antenna, that is to say here in the material.

Depending on the frequency of the signal, the efficiency of the transmission changes.

There are therefore frequencies for which transmission is more efficient.

These frequencies depend on the geometry of the antenna but also the dielectric constant (relative permittivity) of the material near the antenna (for example concrete or mortar with all its constituents including water).

Thus, the transmission frequencies are directly related to the water content of the material, but also to the behavior of the water at the measurement frequency.

It is then possible to discern the different types of bonds of the molecules water with the material, and therefore to analyze more finely the moisture content and the progress of drying.

The measuring apparatus therefore receives, in response to the transmitted signal, a reflected signal.

This reflected signal contains all the information relating to this water content: what is not transmitted in the material is reflected by the antenna.

There is therefore a negative peak in the signal reflected at the frequencies where the signal is transmitted.

The attenuation exhibited by the reflected signal with respect to the radiofrequency signal emitted is generally expressed as a complex number. The frequency analysis can be performed for a zero phase shift between the reflected signal and the transmitted signal, which corresponds to a usual case of using a reflectometer. It will be observed that it is also possible to have a non-zero phase shift between the reflected signal and the transmitted signal, which shifts the determined resonant frequencies. Such an offset of the resonance frequencies does not prevent the interpretation of the measurements for estimating the moisture content.

In a simple embodiment, the reflected signal is received and analyzed in frequency with a zero phase shift compared to the transmitted signal.

Advantageously, the frequency analysis of the reflected signal comprises a calculation of reflection coefficients of the reflected signal to determine the resonant frequencies.

Advantageously, the method according to the present invention comprises a final step during which, when the measured humidity level is equal to or greater than a predetermined threshold humidity level, the interface between the antenna and the measuring apparatus if it is wired, is cut, preferably flush with the material.

In an application for the construction industry, particularly the building industry, once the target water content for the concrete (or for the mortar) is reached, the interface possibly exceeding concrete (or mortar) may be cut off to go to the next step in the realization of the work.

The antenna is then lost in the material (for example in concrete or mortar).

Given the cost of such an antenna, this is quite acceptable compared to the existing solutions described above.

The measurement method proposed in the context of the present invention is therefore simple and quick to implement; the person in charge of measuring the moisture content of a material such as for example concrete or mortar does not need to have special qualification because it is enough for him to cooperate the measuring device with the antenna, then read the result displayed on the device.

It is thus understood that the measurement method proposed in the context of the present invention is particularly interesting for the construction industry (and in particular the construction industry): it allows the operator to measure, quickly and several times if necessary, the moisture content of a material such as concrete or mortar; such a method allows him to effectively manage drying times and decide whether or not the continuation of the work.

The object of the present invention relates, according to a second aspect, to a non-destructive measurement method as described above for measuring the moisture content of a solid material comprising one or more inorganic binders.

Advantageously, the inorganic binders are in particular chosen from cement, lime or calcium sulphate.

The solid material targeted by said use is particularly advantageously a concrete or a mortar during drying.

Advantageously, when the material is a mortar being dried, the resonance frequencies greater than or equal to 1.8 GHz are considered to characterize a presence of water of crystallization in the mortar, while the resonance frequencies strictly less than 1 , 8 GHz are considered to characterize a presence of residual water in the mortar.

According to a very particularly preferred variant of this same use, it is preferable to use said solid material as part of the floor structure of a residential or professional premises, for the laying of a layer of flooring. on said structure.

The term "ground structure" designates the structure comprising the concrete slab resulting from the structural work corresponding to the construction of the building, said slab being preferably covered with a mortar screed and / or a layer of smoothing obtained by pouring liquid mortar. The leveling layer is intended to level the surface defects of the underlying layer (concrete slab or mortar screed), to obtain a flat and smooth surface, suitable for receiving a floor covering.

Among the floor coverings which can be laid, preferably by gluing, on said structure, there may be mentioned, for example, and without limitation, a floor, a tile, a flexible coating such as knitted carpet, tufted, woven , flocked, in strips or slabs, needle-punched, strip or slab flooring, homogeneous or heterogeneous polyvinyl chloride-based floor, polyvinyl chloride-based floor covering on jute or polyester backing or polyester backing with polyvinyl chloride backing, polyvinyl chloride-based flooring on a foam, a polyvinyl chloride-based floor covering with a cork base, an expanded polyvinyl chloride-based floor covering, a semi-flexible polyvinyl chloride-based slab or a chipboard slab. cork with wear layer based on polyvinyl chloride.

According to this latter embodiment, the implementation of the solid material in the form of a concrete slab or a mortar screed, or even a leveling layer, is particularly advantageous.

The object of the present invention relates, according to a third aspect, to a non-destructive measurement method as described above for measuring the moisture content of a material chosen from:

natural soil,

a solid material containing no inorganic binder and comprising one or more organic binders,

a composite solid material obtained by sintering,

a powdery material,

wood, for example in the form of chips, sawdust, granules or chips, or

a liquid material.

As mentioned in the preamble of the present description, other uses of the method according to the invention can also be envisaged in the context of the present invention.

Correlatively, the object of the present invention relates, according to a fourth aspect, a non-destructive measuring system for measuring a moisture content of a material, the system being configured to allow the implementation of the various steps described herein. -above.

More particularly, the system according to the present invention comprises:

- a measuring device of the reflectometer type;

an antenna embedded in the material, and

an interface between the antenna and the measuring device.

The system meter is configured to:

- emit a radiofrequency signal in the material through the antenna; receiving a reflected signal in response to the radiofrequency signal transmitted;

frequency analysis of the reflected signal to determine resonant frequencies of the antenna in the material;

- estimate the moisture content of the material from the determined resonant frequencies.

Preferably, the antenna is of the dipole type.

In an advantageous embodiment, the antenna is of wired type and comprises two segments which each extend in the material between a proximal end and a distal end with respect to the interface with the measuring apparatus, the two segments defining vectors having between them a negative dot product.

The two segments thus form an antenna of the lambda type out of two.

In one variant, the segments are coplanar.

Preferably, this plane is the median plane; this median plane corresponds here to the plane which extends in the median part of the material (that is to say the plane extending in the central layer of the material, at heart).

In another variant, the segments are respectively in planes which extend longitudinally in the direction of the material.

In the same way as before, each of these planes extends substantially in the middle part of the material.

In one case as in the other, the fact that the segments extend in the middle part of the material makes it possible to measure closer to the core of the material.

Preferably, the segments of the antenna each have identical dimensions (length, diameter).

Advantageously, the two segments extend in substantially opposite directions. In addition, they can each be of a length of between 5 to 15 centimeters, preferably substantially equal to 7.5 centimeters.

Advantageously, the antenna is isolated from the material by a protective sheath, preferably an epoxy resin.

It is preferable that the antenna be isolated by a neutral resin so as to make it insensitive to its environment, for example concrete (or mortar) and all the chemical reactions that take place there.

Advantageously, the interface between the antenna and the measuring device, if it is wired, has a connector configured to connect to the measuring device of the reflectometer type. Preferably, the interface is then configured to be removable, for example via a breakable section in the form of precut or section thinning.

Advantageously, it is possible to provide that the system comprises a removable float adapted to be temporarily fixed to the interface protruding from the material; this float is configured to adjust the positioning of the antenna in the material, for example during the casting of the material and / or when it is in the liquid state.

Such a float is particularly advantageous for correctly positioning the antenna with respect to the material to be tested, in particular for positioning it during the pouring of the fresh concrete (or mortar) on a horizontal ground, for example during the pouring of a concrete slab. , a mortar screed, or a layer of patching.

Thus, the present invention, by its different structural and functional technical characteristics, allows to measure simply and accurately the moisture content of a material such as for example a concrete or a mortar during drying.

The measurement is non-destructive and is made to heart by the presence of an inexpensive sensor.

Brief description of the attached figures

Other characteristics and advantages of the present invention will emerge from the description below, with reference to the appended FIGS. 1 to 6, which illustrate an embodiment of this embodiment devoid of any limiting character and in which:

FIG. 1 represents a schematic view of a system according to an exemplary embodiment with a meter of the reflectometer type connected to a dipole type antenna embedded in a mortar screed;

- Figure 2 shows a sectional view of an interface section between the antenna and the measuring apparatus;

FIGS. 3a to 3h are graphs, derived from experimental results, each representing the evolution of the reflection coefficients of the signal reflected as a function of a frequency ranging from 100 MHz to 3GHz, at different times during the drying phase of the mortar ;

FIGS. 4a, 4b and 4c are graphs showing the evolution during the first 30 days of drying of the frequency of each of the frequency peaks observed for the reflection coefficients;

Figures 5a, 5b and 5c are graphs represent the evolution of the frequency of each of the three peaks observed and the evolution of the mass of the mortar during the first hours of drying;

Figure 6 is a flowchart illustrating the method according to an exemplary implementation.

Detailed description of an advantageous embodiment

A non-destructive measurement according to an advantageous exemplary embodiment as well as the associated system will now be described in the following with reference to Figures 1 to 6.

The example described here concerns the monitoring of the moisture content of a mortar in the form of a screed.

It will be understood that this is a purely illustrative example which in no way has a limiting character.

Indeed, as stated above, other examples of implementation for materials other than the mortar may be considered in the context of the present invention.

It is known that a drying phase is necessary for a mortar screed to achieve mechanical characteristics compatible with the standards of the construction, in particular to be able to lay a coating layer on said screed.

Until now, this phase is controlled by complex measurements made on samples: for example a destructive measurement of the chemical type using calcium carbide or a measurement of weight.

In the example described here, designing a system avoiding these samples and allowing an accurate measurement, inexpensive and simple to perform, to determine the moisture content of a mortar being dried is one of the objectives of the present invention. .

On the one hand, this measure must be non-destructive, especially to meet the requirements of the building industry.

On the other hand, this measure must be done at heart, not on the surface, to be the most precise.

This is made possible in the context of the present invention by the design of a sensor 100, also called lost sensor.

As mentioned above, in order to determine the dielectric properties of a material such as mortar and thus to know its moisture content, it is possible to carry out low frequency measurements, for example by performing measurements by effect capacitive. This is not very satisfactory.

The concept underlying the present invention is based on a high frequency measurement using a microwave antenna, preferably dipole type.

In the example described here, the sensor used 100 comprises an antenna composed of two son 10 and 10 '.

During a preliminary step S0, this sensor 100 is partially inserted into the mortar screed B during the laying, here during the casting of said screed.

Following this step S0, the two son 10-10 'of the antenna each have:

two segments 10a-10a 'which form the antenna proper, which are embedded in the mortar B, and

an interface 10b-10b ', 40 between the antenna (the two segments 10a-10a') and the measuring apparatus 200, the interface protruding here from the mortar B.

In the example described here, the segments 10a and 10a 'each extend horizontally respectively along planes P and P'.

These planes P and P 'are substantially parallel to each other.

More particularly, in this example, the planes P and P 'are merged: the first portions 10a and 10a' are therefore coplanar.

The segments may be arranged according to other arrangements. In particular, the segments which each extend in the material B may be arranged in such a way that vectors defined between a proximal end and a distal end of the two segments with respect to the interface with the measuring apparatus define vectors having between them a negative scalar product, that is to say that they have an angle between them between π / 2 and 3π / 2.

In the example described here, the segments 10a and 10a 'extend in the middle part of the mortar screed B so as to allow a measurement at heart.

In this example, the interface 10b-10b ', 40 between the antenna and the measuring device 200 comprises a specific connector 40 which makes it possible to connect the segments 10a, 10a' of the antenna to the reflectometer 200 via the wires of connection 10b, 10b '.

More particularly, in the example described here, the connector 40 is configured to allow the reflectometer 200 to be connected via a coaxial cable, one being connected to the central core of the wires 10, 10 ' and the other to the mass of the threads.

According to other possible embodiments, the interface between the antenna and the measuring device can be performed wirelessly, for example via a near-field communication (NFC) between the antenna and the measuring device. To promote the positioning of the antenna in the mortar B, when this antenna is wired with an interface also wired, it is provided according to the invention to use a float 30.

More particularly, this float 30 makes it possible to adjust the relative height of the segments 10a and 10a 'of the antenna in the mortar B.

In this example, the float 30 is removable and is fixed on the interface 10b-10b '. The float 30 allows the positioning and positioning of the son 10 and 10 'in the mortar screed B during casting.

Once the son 10 and 10 'are positioned, the float 30 is removed.

In the example described here, the segments 10a and 10a 'each depart in opposite directions so as to form a lambda type dipole antenna out of two.

It is preferable that these wires 10 and 10 'are isolated.

In the example described here and illustrated in FIG. 2, there is provided a protective sheath 20 and 20 'for each of the wires 10 and 10'.

This sheath 20, 20 'is composed at least partially in a neutral resin so that the son 10 and 10' are insensitive to the mortar B and all the chemical reactions that take place therein.

The principle underlying the present invention is to send electromagnetic energy into the son 10 and 10 ', and measure the radiation losses of these son 10 and 10'.

In the center, the two son 10 and 10 '(at the interface 10b-10b') therefore leave the mortar B so as to be connected with the appropriate portable measuring apparatus 200; this apparatus 200 is of the reflectometer type.

In the example described here, once the reflectometer 200 connected to the sensor 100, the operator sends in the sensor 100 a radiofrequency signal S_RF from the reflectometer 200, this during a transmission step SI.

A part of this transmitted signal S_RF is then reflected towards the measuring apparatus 200 (referred to as a reflected signal, denoted SR), while another part of this signal S_RF is transmitted in the environment of the sensor 100, in particular in the mortar B (we speak of transmitted signal, noted ST).

The measuring apparatus 200 then receives this reflected signal SR during a step S2.

The frequency at which the energy emitted by the signal S_RF is best transmitted in the mortar B depends on the electromagnetic properties (in particular of the relative permittivity) of the mortar B in the vicinity of the dipole 100. This frequency depends directly on the amount of water contained in the mortar

B.

Indeed, as mentioned above, the frequencies of the transmitted signal ST are directly related to the water content of the mortar B, but also to the behavior of the water at the frequency of the measurements.

The correlation between the resonance frequency and the moisture content of a material such as the mortar is deduced from the following formulas:

It is known first of all that the resonance frequency is a function of the propagation velocity νφ of the electromagnetic waves and the length L of the antenna formed by the segments 10a and 10a ':

v <p

'uence resonance = -

2L

The velocity of propagation νφ of the electromagnetic waves in the material is given by:

where the variable £ _r is the relative permittivity of the mortar.

The relative permittivity of the dry mortar is of the order of 2 to 7, while the relative permittivity of the water is about 80.

Of the two formulas above, we have the following relation between the resonant frequency and the moisture content of the mortar:

Frequency _ resonance =

From the measured resonant frequency and the aforementioned formulas, it is possible to calculate £ _r which is the relative permittivity of the mortar in which the antenna is embedded. In a manner known in itself, the dielectric permittivity of a material makes it possible to establish the moisture content of this material. Therefore, the calculated permittivity £ _r according to the measured resonant frequency to determine the moisture content of the test mortar.

The reflected signal SR, accessible directly from the measuring apparatus 200, therefore contains all the information relating to the water content of the mortar B: indeed, if the signal is transmitted by the wires 10, 10 'to the mortar B, it is not reflected to the device 200.

The reflected signal can in particular be received and analyzed in frequency with a zero phase shift compared to the transmitted signal. It is then sufficient to calculate the reflection coefficients specific to the reflected signal to obtain the resonant frequencies.

In the example described here, the frequency response is recorded over time by a frequency sweep between 10 MHz and 3 GHz, with the reflectometer 200 or a network analyzer.

Following this measurement, the evolution of the reflection coefficient of the SR signal during the scanning (i.e. according to the frequency variation) is calculated, this at several moments during the drying.

FIGS. 3a to 3h are derived from experimental results and represent the reflection coefficients of the reflected signal SR as a function of a frequency ranging from 100 MHz to 3GHz, respectively:

the first day of drying (Figure 3a),

the second day of drying (Figure 3b),

the third day of drying (Figure 3c),

the fifth day of drying (Figure 3d),

the tenth day of drying (figure 3e),

the fifteenth day of drying (Figure 3f),

the twentieth day of drying (Figure 3g), and

the thirtieth day of drying (Figure 3h).

As illustrated in FIG. 3a to 3h, a negative peak appears in the reflected signal SR for the frequencies in which the signal S_RF is "the best" transmitted in the mortar B.

In these figures, there is a first PICl peak around 500MHz, a second PIC2 peak around 1GHz and a third PIC3 peak around 2.3GHz.

It will be noted here that, in the first hours (see FIG. 3a), it is also possible to distinguish a fourth PIC4 peak; this peak PIC4 then disappears.

Each of these peaks PIC1, PIC2, PIC3 and PIC4 thus corresponds to the resonant frequencies of the antenna formed by the segments 10a and 10a 'in the mortar B.

The resonance frequency depends directly on the relative permittivity of the material, and therefore on the moisture content of the material.

It is further observed that as a function of time, the three resonance frequency peaks PIC1, PIC2, PIC3 move towards the high frequencies.

FIGS. 4a, 4b and 4c exploit the above results and represent the evolution of the frequencies of each peak PIC1, PIC2 and PIC3 (resonance frequency) during the 30s. first days.

After analysis, the evolution of the resonance frequencies is frank and follows very well the evolution of the water content in the mortar during drying.

Note in particular in these figures 4a, 4b and 4c a break of each curve after about ten hours of drying: this breaking point corresponds to a decrease in the amount of water that evaporates.

In concomitance with this breaking point, it is also observed that each of the frequencies marks a change: the amplitude and the direction of variation are different according to the observed peak.

The curves of FIGS. 4a and 4b relating to the first two observed resonant frequencies (corresponding to the first two peaks) evolve in a continuous and almost similar manner.

The curve of FIG. 4c which relates to the third resonant frequency observed (corresponding to the third peak) shows a strong variation during the first five days; then, this curve shows a stabilization of this frequency.

If we compare the results of these measurements with known measurement results such as weight measurements, we can see that the evolution of these resonant frequencies is the opposite of the evolution of the weight of the mortar.

This is confirmed by the graphs of FIGS. 5a, 5b and 5c which show the evolution of the frequency of each of the three peaks PIC1, PIC2 and PIC3 and the evolution of the mass of the mortar B during the first hours of drying.

The analysis of these graphs makes it possible to deduce that there is a direct link between the evolution of these frequencies and the evolution of the weight.

Frequency analysis also gives very relevant information: the harmonics of the dipole resonances seem to provide interesting information on the nature of the water present in the mortar.

At the beginning of drying, there is a strong evaporation of water until solidification of the mortar (after two to six hours of drying).

After a few days, the water is almost evaporated. The frequencies of the last peak hardly change after six days of drying.

Thus, the studies carried out made it possible to observe that:

frequencies equal to or greater than 1.8 GHz give information on changes in the water of crystallization;

frequencies below 1.8 GHz give information on residual water (that is, the water that evaporates from the mortar).

After calculating the reflection coefficients and determining the resonance frequencies, a frequency analysis step S3 is thus provided which makes it possible to accurately estimate the moisture content of the mortar B.

This analysis also makes it possible to determine the nature of the water present in the mortar:

residual water, or

water of crystallization.

When the estimated humidity rate in this step S3 reaches a predetermined threshold rate in accordance with the standards in force, then the operator can cut the interface 10b, 10b 'protruding from the mortar screed B.

This corresponds to the final step S4 of the measurement; the work can continue. A first advantage is that the measurement proposed here in the context of the present invention is non-destructive: it is therefore no longer necessary to take samples in the mortar.

As a result, the measurements made here according to the present invention can be multiplied to precisely follow the evolution of the water content of the mortar.

A second advantage is that the sensor 100 is simply composed of son 10 and 10 ', as thin as necessary, and therefore it is not invasive while remaining non-local since the measurement is done over the entire length of the son 10 and 10 'and in particular the length of the segments 10a and 10a'.

A third advantage is to be able to analyze very finely the nature of the water contained in the mortar B: the measurement proposed in the context of the present invention gives in addition additional information on the nature of the water contained in the mortar.

Indeed, as explained in the preamble of the present description, only a portion of the water in the mortar must leave; the rest is part of the composition of the dry mortar; thus, with the measurement, one distinguishes in the reflected signal:

frequencies equal to or greater than 1.8 GHz which give information on changes in the water of crystallization;

frequencies below 1.8 GHz that provide information on residual water (ie water that evaporates from the mortar).

One of the many other advantages of the measurement proposed here in the context of the present invention is to have an extremely short measurement time: it is sufficient to the operator responsible for carrying out the measurement of connecting the reflectometer 200 to the portion of the sensor 100 protruding from the mortar B and reading on the measuring apparatus 100 the results without performing manipulations.

This allows, in constant intervention time, to multiply the measurements and to optimize the progress of the construction.

It will also be noted that, thanks to the invention, it is no longer necessary to introduce a resonator into the reflectometer.

It will be noted, however, that one of the disadvantages of the present invention is to have an antenna lost in mortar screed B.

However, the antenna used here is composed only of son 10 and 10 ': its cost is very low compared to the hourly cost of a technician to perform a measurement of the type chemical measurement using calcium carbide.

Note also that leaving the antenna in the mortar screed B is not unfavorable from the point of view of the structure.

The present invention thus provides actors in the construction industry, and particularly the building industry, with an easy-to-use technology for measuring and monitoring the evolution of the moisture content of a material B such as, for example, the mortar or concrete being dried.

The technology proposed here also requires little equipment: a simple system 300 consisting of a reflectometer 200 and a sensor 100 instrumenting the material B to be tested is sufficient.

It should be observed that this detailed description relates to a particular embodiment of the present invention, but in no case this description is of any nature limiting to the subject of the invention; on the contrary, its purpose is to remove any imprecision or misinterpretation of the claims that follow.

Claims

A method of non-destructive measurement of the moisture content of a material (B), comprising:

the emission (SI) of a radiofrequency signal (S_RF) in the material (B) through an antenna (10a, 10a ') embedded in the material;

the reception (S2) of a reflected signal (SR) in response to the radiofrequency signal transmitted (S_RF);

frequency analysis (S3) of the reflected signal for determining resonance frequencies of the antenna (10a, 10a ') in the material (B);

the estimation of the moisture content of the material (B) from the determined resonant frequencies.

2. The method of claim 1, wherein the reflected signal (SR) is received and analyzed in frequency with a zero phase shift compared to the transmitted signal.

The method of any of the preceding claims, wherein the frequency analysis of the reflected signal comprises calculating reflection coefficients of the reflected signal to determine the resonant frequencies.

4. Use of a non-destructive measurement method according to any one of claims 1 to 3 for measuring the moisture content of a solid material (B) comprising one or more inorganic binders. 5. Use according to claim 4, wherein the solid material is a concrete or a mortar being dried.

6. Use according to claim 5, wherein, when the material (B) is a mortar being dried, the resonance frequencies greater than or equal to 1.8 GHz are considered to characterize a presence of water of crystallization in said mortar, and the resonance frequencies strictly below 1.8 GHz are considered as characterizing a presence of residual water in said mortar

7. Use according to one of claims 4 to 6, wherein the solid material (B) is implemented as part of the floor structure of a residential or professional premises, for the laying of a layer of flooring on said structure.

Use according to claim 7, wherein the solid material (B) is implemented in the form of a concrete slab or a mortar screed, or even a leveling layer.

Use of a non-destructive measuring method according to any one of claims 1 to 3 for measuring the moisture content of a material (B) chosen from:

- natural soil,

a composite solid material obtained by sintering,

a powdery material,

- wood or

a liquid material.

A non-destructive measurement system for measuring a moisture content of a material (B), the system (100) comprising:

a measurement apparatus (200) of the reflectometer type;

an antenna (10a, 10a ') embedded in the material (B); and

- an interface between the antenna and the measuring device,

the measuring apparatus being configured to:

- transmitting a radiofrequency signal (S_RF) in the material (B) through the antenna (10a, 10a ');

receiving a reflected signal (SR) in response to the transmitted radiofrequency signal (S_RF);

frequency analysis of the reflected signal to determine resonance frequencies of the antenna (10a, 10a ') in the material (B);

- estimate the moisture content of the material (B) from the determined resonant frequencies.

The system of claim 10, wherein the antenna is wired and comprises two segments (10a, 10a ') which each extend in the material (B) between a proximal end and an end distal to the interface with the measuring apparatus, the two segments defining vectors having between them a negative scalar product.

The system of claim 11, wherein the two segments extend in substantially opposite directions.

13. System according to any one of claims 10 to 12, wherein the antenna (10a, 10a ') is isolated from the material (B) by a protective sheath (20, 20'), preferably consisting at least partially of epoxy resin.

14. System according to any one of claims 10 to 13, comprising a removable float (30) attached to a portion (10b, 10b ', 40) of the interface protruding from the material (B) and configured to adjust the positioning of the the antenna in the material (B), for example during the casting of the material.